1. Field of the Invention
The present invention relates to a packet retransmission method, and more particularly, to a method of re-transmitting packets in a radio communication system having a plurality of antennas.
2. Background of the Related Art
Lately, the third generation mobile communication system, to which many efforts and developments are made, supports multimedia services as well as voice services of the previous mobile communication system. Specifically, the multimedia services are essential to Internet spreading worldwide. In order to support high-speed multimedia services capacities of uplink and downlink should be greatly increased. The downlink capacity is determined by maximum transmission power used by a cell site, i.e. base station. Hence, if we can reduce required SNR for calling of all of the mobile terminals existing in a calling area of the base station, the downlink capacity can be increased. The uplink capacity can be increased by Space Diversity, Multi-user Detector, etc. Theses methods are applied to the uplink because a reception device of the base station has an allowable limitation of complexity larger than that of the mobile terminal and also a power consumption limitation smaller than that of the mobile terminal.
Such a method as space diversity, multi-user detector and the like has difficulty in being applied to the mobile terminal to increase the downlink capacity. The size and power consumption of the mobile terminal and the system complexity set limitations to the application of the downlink, i.e. reception algorithm of the mobile terminal. Many efforts are made to the method of increasing downlink communication capacity by increasing complexity of a transmitter instead that of a receiver. One of such efforts is Transmit Diversity.
Transmit Diversity provides multi-paths between the transmitter and receiver by multiple antennas in a transmitter side of the downlink, thereby enabling a diversity gain generated from using Reception Diversity. Such a method increases communication capacity over the downlink without increasing the complexity or size of the mobile terminal to improve communication performance.
Asynchronous WCDMA (wideband code division multiple access) standards of 3GPP (3rd generation partnership project) as one of 3rd generation mobile communication standards include two kinds of Transmit Diversity technologies. One is an open loop transmit diversity type and the other is a closed loop transmit diversity type. STTD (space time transmit diversity) and TSTD (time switched transmit diversity) belong to the open loop transmit diversity type, and CL1 (closed loop mode 1) and CL2 (closed loop mode 2) belong to the closed loop transmit diversity type. The open loop mode is that the transmitter does not use information of channel state, which changes a transmission antenna periodically using multiple transmission antennas or uses simple encoding. The closed loop mode use feedback information of channel state to calculate the weight of an antenna for transit diversity. It is known that the closed loop mode, compared to the open loop mode, receiving information of the channel state from the receiver has more improved performance.
In STTD or TSTD, a transmission antenna is added to a conventional single transmission antenna system. In STTD, a transmitter equipped with two (or four) transmission antennas separately encodes and modulates data of the same signal source, transmits them through the respective antennas, and a receiver then combines to demodulate them to improve reception performance.
TSTD is a method of transmitting data by switching a pair of transmission antennas periodically, which is adopted for downlink transmit diversity of a system following UMTS (universal mobile telecommunication system) standards as a European IMT-2000 mobile communication system.
TSTD of the UMTS standards is applied to 1.28 Mcps TDD (time division duplex, which performs transmission and reception by time division). TSTD performs switching operation so that antenna transmitting data each 5 ms sub-frame as transmission unit of 1.28 Mcps TDD is switched.
A configuration of a TSTD system defined in UMTS standards of 3GPP is shown in FIG. 1.
Referring to FIG. 1, a front part of a TSTD system is a structure of a general transmission signal processing part existing in a conventional single antenna system, and a rear part is a TSTD device part.
A signal (or data) processed through the front part of the TSTD system is switched to be transmitted to respective antennas ANT1 and ANT2 over DSCH (downlink shared channel).
Packet data service adopts a technique of using correction codes and a technique of requesting retransmission for the reliable data transmission. Various techniques of requesting retransmission by detecting error at a reception part are explained as follows.
First of all, there is ‘simple information feedback’ which is performed in the following manner. Once information received by a receiver is sent back to a transmitter, the transmitter detects any error in the corresponding information and the corresponding information is then retransmitted if there exists errors.
Secondly, there is ‘ARQ (automatic repeat request)’, in which a receiver checks any error in transmitted data and request a retransmission of the corresponding erroneous data to the transmitter if there exists errors.
Thirdly, there is ‘parity check’, which includes ‘error detection coding’ of checking any error in the transmitted data. And the receiver executes error check by using redundant 1 check bit.
Fourthly, there is ‘cyclic redundancy check (CRC)’ of adding CRC code by block unit to transmit.
The ARQ technique is grouped into stop-and-wait ARQ, continuous ARQ, and adaptive ARQ types.
The stop-and-wait ARQ is performed in a manner that a transmitter transmits one data block and waits until response signal is arrived in the receiver. The receiver transmits ‘negative acknowledgment (hereinafter abbreviated NACK)’ if error is detected in the transmitted data block. If error is not detected in the transmitted data block, the receiver transmits ‘acknowledgment (hereinafter abbreviated ACK)’ to the transmitter. The transmitter transmits a next block if receiving the ACK signal from the receiver, or retransmits the corresponding data block if receiving the NACK signal no response until a predetermined time passes.
The continuous ARQ type is for compensating a disadvantage of the stop-and-wait ARQ type and is grouped into ‘Go-Back-N ARQ’ and ‘Selective ARQ’. The Go-Back-N ARQ is carried out in a manner that, once NACK signal is transmitted from a receiver, a transmitter retransmits all blocks thereafter as well as a corresponding error block. The selective ARQ is a technique of retransmitting the block corresponding to the NACK signal only.
The adaptive ARQ is a method of adjusting a length of a block dynamically to increase transmission efficiency, in which a receiver transfers an error rate to a transmitter then the transmitter adjusts the length of the block appropriately to transmit. Therefore, the adaptive ARQ type has good transmission efficiency.
The above-explained ARQ methods are equivalently applied to a radio packet transmission system as well. Even though various ARQ types are applicable to the radio packet communication system, the receiver should transmit the ACK or NACK signal to the transmitter basically and each of the ACK and NACK is defined by 1 bit. Namely, if the receiver transmits ACK signal, e.g., 1, of 1 bit, the transmitter judges that the transmitted packet is correctly received. If the reception party transmits NACK signal, e.g., −1, the transmitter judges that the receiver fails to receive the packet correctly and then retransmits the corresponding data packet.
FIG. 2 shows the conventional antenna switching pattern of a TSTD system. Referring to FIG. 2, a length of a sub-frame of 1.28 Mcps is 5 ms. First and second antennas ANT1 and ANT2 alternately transmit sub-frames, respectively. Yet, slots of each sub-frames are transmitted though the same antenna.
As mentioned in the foregoing explanation, a receiver in a TSTD system performs an error check to transmit ACK or NACK response to a transmitter. If the response is ACK, the transmitter transmits a following frame. If the response is NACK, a corresponding error frame is retransmitted. In this case, a antenna switching (or selection) pattern (hereinafter called antenna switching) of the related art TSTD is sequential and repeatable. Namely, not considering channel state of each antenna regardless of the retransmitted frame or new frame, the transmission is performed through the antennas switched to each other by a predetermined time interval (sub-frame unit).
FIG. 3 is a diagram of an example of antenna assignment according to a related art.
Referring to FIG. 3, it is assumed that transmission delay time is one sub-frame (5 ms). Namely, after ith sub-frame sub-frame_i has been transmitted, a response signal ACK or NACK of a receiver for an (i−1)th sub-frame sub-frame_i−1 is received. If the response signal is NACK, (i−1)th sub-frame sub-frame_i−1 is retransmitted, If the response signal is ACK, (i+1)th sub-frame sub-frame_i+1 is retransmitted.
An antenna assignment method according to a related art performs according to TSTD technique of having a pair of antennas ANT1 and ANT2 transmit sub-frames sub-fram—0 to sub-frame_n alternately.
Namely, if sub-frame—1 is transmitted to the first antenna, sub-frame—2 is transmitted to the second antenna, sub-frame—3 is transmitted to the first antenna, sub-frame—4 is transmitted to the second antenna, etc. Such operation is repeated until all the sub-frames are transmitted.
The above-explained antenna assignment method is re-explained provided errors exist in transmitting sub-frame—1 and sub-frame—3.
Having transmitted sub-frame—1 to the first antenna, the transmitter transmits sub-frame—2 to the second antenna. After transmission of sub-frame—2, the transmitter receives a signal F1/NACK informing of error existence. Once the signal F1/NACK is received, the transmitter transmits retransmission frame sub-frame—1′ of sub-frame—1 as a next frame of sub-frame—2 to the first antenna. Thereafter, the transmitter transmits sub-frame—3 to the second antenna and then transmits sub-frame—4 to the first antenna.
After transmission of sub-frame—4, the transmitter receives a signal F3/NACK informing that error exists in sub-frame—3. Once the signal F3/NACK is received, the transmitter transmits retransmission frame sub-frame—3′ of sub-frame—3 to the second antenna as a next sub-frame of sub-frame—4.
In fading environments where the channel state slowly varies, the TSTD system having ARQ applied thereto enables to maintain independency between retransmission sub-frames using space diversity by switching transmission antennas.
However, the antenna switching of the related art TSTD system is performed sequentially and repeatedly regardless of the channel states of the two antennas ANT1 and ANT2, whereby it is unable to consecutively use the antenna having a better channel state.